The present invention relates to a switching apparatus for a horizontal driving pulse which is input into a horizontal deflection circuit in a television receiver.
Broadcast satellite (BS) digital broadcasting has ushered in the use of a television (TV) receiver in recent years, and importance is placed on development of the capability that the TV is provided for various kinds of input signals. A conventional phase control device, which controls a horizontal driving pulse in a horizontal deflection circuit, is described as follows.
FIG. 3 is a block diagram of a conventional phase control device which controls a horizontal driving pulse. Frequency discriminator 1 discriminates a type (e.g. an interlace signal, or a progressive signal) of input video signal (not shown) from a frequency of input horizontal synchronization (hereinafter referred to as horizontal sync, or H. Sync) signal A. Reference phase signal generator 2 outputs reference phase signal B which provides a phase of a horizontal deflection pulse optimized for the input video signal responsively to horizontal sync signal A and discrimination result J of frequency discriminator 1 (an output signal of discriminator 1). The phase of the horizontal deflection pulse determines the retrace-time of a scanning line. Timing signal generator 3 generates timing signal C, which is responsive to the input video signal, based on the horizontal sync signal A and the discrimination result J of the frequency discriminator 1. Sawtooth wave generator 4 outputs sawtooth wave signal D which has a phase responsive to timing signal C. Multiplier 6 multiplies reference phase signal B by an output signal of comparator 5, and outputs multiplier output G. Low-pass filter (LPF) 7 makes multiplier output G smoothed into average voltage Vav, which is a DC voltage, and outputs average voltage Vav. Comparator 8 compares sawtooth wave signal D with average voltage Vav, and outputs phase control signal H. Signal H is at a high level when sawtooth wave signal D is larger than average voltage Vav, and is at a low level in the other period. Horizontal driving pulse generator 20 outputs horizontal driving pulse I. Pulse I is synchronized with a rising edge of phase control signal H, and has a duty cycle which is determined by discrimination result J of frequency discriminator 1. Horizontal deflection circuit 13 receives horizontal driving pulse I, and outputs horizontal deflection pulse E. Comparator 5 compares horizontal deflection pulse E with a specified reference voltage, and generates signal F. Signal F is at a high level when a voltage of horizontal deflection pulse E is equal to or larger than the reference voltage.
Timing signal generator 3 and sawtooth wave generator 4 constitute a sawtooth wave generator unit. The unit outputs a sawtooth wave signal having a phase responsive to a type of input video signal which is extracted from the horizontal sync signal. Comparator 5, multiplier 6, and LPF 7 constitute a phase difference voltage detector. The detector outputs a phase difference voltage which is the DC voltage (average voltage Vav) responsive to a phase difference between reference phase signal B and horizontal deflection pulse E.
Furthermore, comparator 8 compares sawtooth wave signal D with phase difference voltage Vav, and outputs a phase control signal, which is used for the horizontal driving pulse to have a phase corresponding to the phase difference voltage. Alternative signal generator 14 generates and outputs an alternative signal with free running clock pulse generated therein. The alternative signal is asynchronous to an output signal of horizontal driving pulse generator 20. Horizontal output controller 16 detects whether an operation of a frequency change accompanying an input video signal change is in progress or not, and whether an operation of a power-on is in progress or not. Switching circuit 15 outputs the alternative signal to horizontal deflection circuit 13 while the operation of the frequency change is in progress or the operation of the power-on is in progress. The alternative signal is alternative to an output signal from horizontal driving pulse generator 20.
The horizontal output controller switches an input signal of horizontal deflection circuit 13 between the output of the horizontal driving pulse generator and the output of the alternative signal generator. The apparatus having components mentioned above may malfunction for various reasons, for example, a signal waveform lacks in shape, a signal waveform loses continuity at the switching operation mentioned above, and so on. A horizontal driving transistor in the horizontal deflection circuit breaks if a waveform response before and after the switching operation is not taken into account.
The present invention relates to a switching apparatus for a horizontal driving pulse which is input into a horizontal deflection circuit outputing a horizontal deflection pulse. The switching apparatus comprises: (a) a horizontal driving pulse generator which produces a horizontal driving pulse from a horizontal synchronization signal; (b) a horizontal deflection circuit which carries out a horizontal deflection with the horizontal driving pulse; (c) an alternative signal generator which outputs an alternative signal which is different from the horizontal driving pulse; (d) a horizontal output control device which determines the time to switch between the horizontal driving pulse and the alternative signal; and (e) a switching device which switches an output pulse with a control signal which is output by the horizontal output control device. The switching device switches only when both the output signal from the horizontal driving pulse generator and the output signal from the alternative signal generator change from a low level to a high level at the same time.
The switching apparatus of the present invention allows the waveform of the horizontal driving pulse not to lack in shape and not to lose continuity at the switching operation. As a result, the output signal waveform of the switching apparatus keeps continuity before and after the switching operation, and the horizontal deflection circuit does not have malfunctions which are a breakage of the horizontal driving transistor and the like.